A logging sonde, or "tool", typically makes measurements radially outwardly in all directions from the tool. This is especially true with a resistivity logging tool. In effect, such a tool without azimuthal focusing measures the bulk resistance of the formation surrounding a well borehole. Changes in the measured resistance are indicative of changes in formation materials penetrated by the borehole, and such changes can be used to determine whether or not the formations contain hydrocarbons within the pore space as opposed to saline water.
Measurements with an induction logging tool are made by forming a field which extends into the formation. This involves forming a field pattern involving one or more coils carried in the induction logging tool. While the coils may be complex arrays, the construction of the arrays, the operating frequencies and other variables involved are merely the backdrop for the present disclosure. More specifically, this disclosure is directed to an aspect of tool positioning so that the data obtained from an induction log can be evaluated. That evaluation is not easily accomplished. The operation and the data obtained from such logging tools must be processed to obtain a useful resistivity log. Such processing includes correcting measured signals for adverse effects of the borehole. One aspect of the processing is involved in the standoff distance between the tool and the penetrated formation.
The tool is lowered preferably on a wireline into an open borehole to make measurements in the formations. The tool can also be operated attached to a drill string, coil tubing, or drill pipe. The borehole is normally filled with drilling fluid, normally water or oil with clay based additives to increase weight. This creates a zone of borehole effects. The zone of borehole effects includes at least the open well borehole. There may be a thin layer of the weight material from the drilling mud which is defined as a mud cake deposited against the wall. The solvent in the drilling fluid will soak into the adjacent formation and thereby change the nature of the formation at least for a little distance into the formation. These borehole effects normally must be removed from the calculations which are carried out in evaluating a resistivity log. One important aspect of the data reduction routine is determining the physical alignment of the logging tool with the open hole in which measurements are made. Even with a small diameter induction logging tool, centralizers are mounted on the logging tool to position it along the centerline axis of the open hole. One typical assumption is that the tool is positioned on the centerline axis. This centerline positioning is an important aspect of running the tool into the well borehole. Briefly, centralizers are mounted along the length of the tool body so that the centralizers move the tool to the desired location.
By contrast, there are a number of logging tools which require that the tool be eccentered or even "jammed" against the sidewall of the open borehole. It is necessary for operation in this sort of tool to have facial contact between the tool and the formation. Typically, the tool will mount some kind of measurement shoe broadly positioned against the sidewall of the well borehole. That position is achieved by decentralizing the tool with a bow spring which curves to the side, or, alternately, by a powered decentralizing caliper arm well known in the art. The bow spring decentralizes the tool and jams it against the formation so that the desired kind of physical coupling is achieved. The bow spring is located at a particular side of the cylindrical tool body so that the shoe contact is achieved. A single tool string comprising several different serially connected elongated tools thus will require a knuckle between the portion that preferably operates on the centerline, and the portion which best operates jammed against the sidewall of the well borehole.
Great efforts are made as noted to centralize an induction logging tool where it is connected in a tool string, but located below tools which operate best when eccentered against the side of the well borehole. Centralizers below the knuckle are required to overcome the positioning of the decentralizing bow springs. Typically, two or three eccentering bow springs or the like are included above the knuckle so that portion of the tool string is jammed to the side. Below the knuckle, the remainder of the tool is intended to be located on the centerline axis of the borehole. There are certain symmetries which result from the collection of data from that location.
With an induction logging tool, it is important to have some spacing between the induction logging tool and the adjacent formations. In the prior art, the induction tool is usually operated centralized or at least at a fixed standoff within a cylindrical well borehole. Sometimes, however, the formations simply do not drill in a perfect circle. The hole formed by the drill bit may collapse partially on one side or the other and create a different shape hole. In some instances, unconsolidated sand may wash away. In many other instances, the drill bit will drill a round hole, but drill collars above the drill bit at a particular location may form a "key seat" because the hole drifts. The circular hole will be distorted to a different shape. As will be understood, many of these problems can arise in lesser or greater amount without any information at the surface indicating that such problem has occurred, or for that matter, that the hole is perfectly cylindrical in shape. The centralizers, however, are included so that the tool is positioned at the idealized fixed distance from the well borehole, even where it is suspended below logging tools preferably eccentered for operation. This controls the tool "standoff", which is defined as the spacing between the outside diameter of the tool and the wall of the well borehole. The standoff is important to assure that predictable and controllable interactions are achieved between the tool and the formation penetrated by the borehole so that the induction log is completed, and the data can be reduced using repetitive assumptions.
In actuality, positioning the induction logging tool at the centerline position of the well borehole decreases the sensitivity of the measured response to resistivity (conductivity) of the formation, but also decreases the sensitivity of the measurement to the resistivity (conductivity) of the material within and immediately adjacent to the borehole. Centralized operation provides more than enough standoff so that the electromagnetic flux in the borehole provides adequate data indicative of formation and borehole effects. If the tool is moved away from the centerline position, sensitivity to the formation is increased provided it is not moved so far that it is jammed against the borehole wall. Sensitivity to borehole effects is also increased. When the tool is against or very near the borehole wall, the measured data is much less reliable and special efforts have to be developed and undertaken in data reduction to remove the distortion arising from that tool position. In many cases, the data is rendered quantitatively unusable. In other words, a standoff that is sufficiently wrong yields data that may not be useful. Restated, when the tool is jammed against the side wall, the data may be loaded with undesirable effects distorting readings of the tool.